Fix a race in iget4. The fs specific data that is used to find an inode
should be initialized while still holding the inode lock.

It adds a 'set' callback function that should be a non-blocking FS
provided function which initializes the private parts of the inode so
that the 'test' callback function can correctly match new inodes.

Touches all filesystems that use iget4 (Coda/NFS/ReiserFS).